Power reserve

Power reserve is how long an automatic watch keeps running once it stops receiving energy, that is, once you take it off. It is measured in hours and depends on how much mainspring fits in the barrel, the small cylinder where the energy is stored, and how quickly the movement uses it.

What the figures mean

• 38 to 42 hours. Normal for automatics under €500. Take it off at night and it is still running in the morning, but leave it for the weekend and it stops.
• 50 to 70 hours. A day and a half to almost three days. It survives a day off the wrist.
• 80 hours or more. More than three days: take it off on Friday evening and it is still running on Monday morning.

Why 80 hours costs less than it used to

For decades, mid-range Swiss watches gave about 38 hours. The Swatch Group’s Powermatic 80 changed that: it lowered the balance’s beat rate, so it uses less energy, and lengthened the mainspring. Today there are 1 watches with 70 hours or more in the catalogue, from €1,240.

The catalogue by power reserve

If you own several watches

With two or three 40-hour automatics, you will usually find them stopped. There are three answers: choose 80-hour models, wind them by hand if they allow it, or use a watch winder, a motorised box that keeps them turning. A winder is convenient but not essential: a stopped automatic does not come to any harm.

Reserve is not accuracy

More hours do not mean the watch gains or loses less time. Accuracy depends on the regulation of the balance and the hairspring, and it usually gets a little worse when the mainspring is nearly empty. That is why it is better to keep it wound than to run the reserve down.